1. Field of the Invention
This invention relates to the monitoring of electrical properties of conductive coatings, and specifically to the monitoring of such properties of conductive optical fiber coatings.
2. Description of the Prior Art
The environment can damage optical fiber. Water and other chemicals can corrode the fiber. Hydrogen can be absorbed by the fiber and cause light attenuation. To protect optical fiber from these and other environmental influences, a thin hermetic coating may be deposited on the fiber in addition to the standard plastic coating. These hermetic optical fiber coatings are typically made of electrically conductive material, electrically semi-conductive material, or they may be made of a combination of materials in which at least one material is electrically conductive. As used herein to describe coatings, the term "conductive" refers to coatings made of electrically conductive material or electrically semi-conductive material and to coatings made of a combination of materials in which at least one material is electrically conductive.
It is common to deposit an essentially non-conductive outer protective coating made of plastic such as acrylate or silicone over the conductive coating. The outer protective coating prevents damage or removal of the conductive coating during handling or service.
It is well known to those skilled in the art that electrical properties of a conductive coating are influenced by the thickness and/or uniformity of the conductive coating over a given area. For example, if the thickness of a conductive coating increases, the electrical resistance decreases. If the thickness of a conductive coating decreases, the electrical resistance increases. A gap in the conductive coating will also cause the electrical resistance to increase. Furthermore, if a conductive coating consists of a mixture of an electrically conductive material and an electrically non-conductive material, a change in the proportion of the materials will affect the resistance of the conductive coating.
It is known that the thickness and uniformity of conductive coatings can be determined by utilizing the relationship between conductive coating thickness and uniformity and the electrical properties of the conductive coating. The measured electrical values are correlated with the particular conductive coating characteristics desired. For example, the resistance of a conductive coating of unknown thickness and uniformity may be measured by a suitable method. The same method may be used to measure the resistances of similar conductive coatings of known thickness and uniformity. The thickness of these similar conductive coatings can be determined by electron microscopy or other methods. The uniformity of these similar conductive coatings can be determined by chemical analysis or magnified visual inspection. The resistance of the conductive coating of unknown thickness and uniformity is then compared to the resistances of the similar conductive coatings of known thickness and uniformity. In this way the unknown thickness and uniformity can be determined. If such correlations are carried out over a variety of conductive coating thicknesses for a particular conductive coating, an acceptable range of resistivity can be determined which will indicate that conductive coating thickness and uniformity is meeting previously set standards for production or quality assurance.
Several methods of monitoring an electrical property of a conductive coating have been used. In one previously known method, two suitable contacts are connected to a conductive coating at a predetermined distance from each other along the axis of the conductive coating. The contacts are also connected to an ohm-meter. A measurement of resistance between the two contacts is taken. If the conductive coating is covered with an essentially non-conductive outer protective coating, a pair of razor blades can be utilized for the contacts. The razor blades slice through the outer protective coating until making contact with the conductive coating. An apparatus for carrying out this previously known method is illustrated in FIG. 1. Razor blades 71 and 73 are separated by a predetermined length at opposite ends of an insulating block 75 and are respectively connected to ohm-meter means 79. Razor blades 71 and 73 are then brought into contact with optical fiber 77 and linear resistance is measured by ohm-meter 79.
Several disadvantages are associated with methods which utilize physical contact with the conductive coating to measure electrical properties. First, if an outer protective coating covers the conductive coating, the outer protective coating must either be penetrated or removed in order to make electrical contact with the conductive coating. Second, the section of the coated optical fiber utilized to make an electrical measurement is usually destroyed by the physical contact which takes place between the conductive coating and the electrical contacts. Such physical contact may produce a nick or scratch in the conductive coating or in the fiber itself. These injuries can produce flaws in the fiber which result in stress concentrations and breakage. As a result, unless electrical properties are to be measured at the end portions of the fiber, the portion of the fiber used to take electrical measurements must be removed and the remaining segments spliced together. Third, methods utilizing physical contact with the conductive coating can only be performed while the fiber is stationary or moving very slowly. This precludes taking measurements while the fiber is in motion, such as during high speed production of the fiber or when the fiber is being transferred from one reel to another.
A contactless method of monitoring an electrical property of a conductive coating is disclosed in U.S. Pat. No. 5,142,228. The method comprises inductively coupling an AC signal to a conductive coating while simultaneously measuring an electrical value dependent upon the electrical resistivity of the conductive coating then positioned within the inductive coil. An example of such an electrical value is Q. The Q of a reactive circuit element, i.e., a circuit having an inductor (L) or a capacitor (C), is defined as the ratio of the element's reactance (X) to its series resistance: ##EQU1##
For an inductor this is given by: EQU Q=X.sub.L /R=.omega.L/R=2.pi.fL/R
where L is the inductance, R is the resistance associated with the inductor, .omega. is angular velocity and f is the corresponding frequency.
An example of an apparatus for carrying out this method is shown in FIG. 2. This particular apparatus can be used for either static or dynamic monitoring. The apparatus includes inductive coil 81 which is electrically connected by leads 83 to Q-meter means 85. Q-meter means 85 incorporates an analogue or digital display meter means 87 for displaying a measured electrical value (Q) in response to the energizing of inductive coil 81.
Another contactless method of inductively measuring an electrical property of an optical fiber coating is disclosed in U.S. Pat. No. 5,013,130, which teaches using a microwave cavity to generate an electromagnetic field around the coated fiber.
Although inductive methods of measuring electrical properties of conductive coatings are non-destructive, they are also associated with several disadvantages. First, the whole circuit must be highly tuned to get a good measurement. Second, as a conductive coating gets thinner and its resistance increases, the electrical values which characterize the reactive circuit become harder to measure. Therefore, a need exists for a non-inductive method of monitoring an electrical property of a conductive coating which may be utilized without contacting the conductive coating.